The present invention relates to a method of and an apparatus for qualitatively and quantitatively analyzing a plurality of trace constituents which are contained in a solution in the form of fine particles or ions, and more particularly to an analytical method and an analyzer suitable for qualitatively and quantitatively analyzing a plurality of trace constituents which are contained in a solution and include a trace constituent low in generation efficiency of photoacoustic signal.
The photoacoustic analysis and the fluorometric analysis have been used to determine a trace constituent contained in a solution. In each of them, a solution containing a trace constituent is irradiated with light (namely, an electromagnetic wave). Further, a photoacoustic signal generated by the light-irradiated trace constituent is detected in the photoacoustic analysis, and luminescence from the light-irradiated trace constituent is detected in the fluorometric analysis. The photoacoustic analysis is described, for example, on pages 865 to 867 of the June 1978 issue (Vol. 50, No. 7) of the "Analytical Chemistry", on pages 650 to 653 of the April 1980 issue ( Vol. 52, No. 4) of the same, and on pages 2275 to 2278 of the November 1986 issue (Vol. 58, No. 11) of the same. Further, the fluorometric analysis is described, for example, in an article entitled "ON-LINE URANIUM DETERMINATION USING REMOTE FIBER FLUORIMETRY" by R. A. Malstrom and T. Hirschfeld ("Analytical Spectrometry", pages 25 to 30). In these publications, however, the use of only either one of the photoacoustic analysis or the fluorometric analysis is described.
A molecule or atom can be excited from a ground state to an excited state by the absorption of light. In many cases, the excited molecule or atom is deexcited to the ground state through a non-radiative or radiative relaxation process. At this time, excess energy is released, as the thermal energy, in the non-radiative relaxation process, and is released, as optical energy, in the radiative relaxation process. A medium containing the above molecule or atom thermally expands on the basis on the thermal energy generated by the non-radiative relaxation, and thus an acoustic wave, that is, a photoacoustic signal is generated. While, fluorescence and phosphorescence are generated by the radiative relaxation process. When the quantum yield of non-radiative relaxation and the quantum yield of radiative relaxation are expressed by .eta..sub.1 and .eta..sub.2, respectively, the following formula is valid:
ti .eta..sub.1 +.eta..sub.2 .perspectiveto..sup.1 (1)
That is, the photoacoustic effect and the fluorescence (or phosphorescence) generated by a substance which has absorbed light, are complementary to each other. Accordingly, a substance which can be detected at high sensitivity by the photoacoustic analysis, has low sensitivity to the fluorometric analysis. While, a substance which can be detected at high sensitivity by the fluorometric analysis, has low sensitivity to the photoacoustic analysis. However, as indicated by the formula (1), the photoacoustic signal and the fluorescence (or phosphorescence ) are simultaneously generated by a substance to be determined, and a ratio of one of the photoacoustic signal magnitude and the fluorescence intensity to the other intensity depends upon the to-be-determined substance. Accordingly, almost all substances can be determined by measuring both the photoacoustic signal and the fluorescence (or phosphorescence). However, a limited number of substances can generate fluorescence or phosphorescence. Accordingly, the fluorometric analysis is applicable to only the above substances. Further, in a case where a first substance incapable of generating fluorescence (or phosphorescence) and a second substance generating strong fluorescence (or phosphorescence) and an extremely weak photoacoustic signal coexist in a medium, it is very difficult to determine each of the first and second substances accurately by only one of the photoacoustic analysis and the fluorometric analysis.
For example, in a case where a chemical species exhibits a plurality of valences, and the luminous efficiency of the chemical species varies with the valence thereof, it is impossible to determine both the concentration of the chemical species and the relative abundances of the above valences by one of the photoacoustic analysis and the fluorometric analysis. In a uranium extraction process in the reprocessing of nuclear fuel, sexivalent uranium U(VI) is extracted from tri-n-butyl phosphate dodecane solution. It is essential for quality control and safeguards that the abundance ratio between sexivalent uranium U(VI) and quadrivalent uranium U(IV) is measured. However, the sexivalent uranium U(VI) exhibits the fluorescence, but the quadrivalent uranium U(IV) does not fluoresce. Accordingly, it is impossible to analyze a solution containing U(VI) and U(IV) by conventional analytical methods.